Recent predictions highlight that the surging demand for computational power could account for a major share of global energy consumption by 2050. To address this sustainability challenge, a paradigm shift toward new, energy-efficient computing architectures is required. The possibility of dynamically tuning material properties via voltage-driven ion migration represents a promising pathway to realize ultralow-power storage and logic devices. In perovskite oxides, the voltage-driven modulation of the oxygen vacancy concentration (δ) ^[1,2] acts as the primary lever to tune these properties. However, the efficiency and thermodynamic trajectory of this oxygen exchange are intrinsically governed by the cation composition. Consequently, understanding how B-site substitution influences the oxygen reduction pathway is critical for designing devices with tailored switching characteristics.

In this work, we present a comparative study between SrFeO_3-δ (SFO) and the B-site substituted SrFe_0.5Co_0.5O_3-δ (SFCO). Using a solid electrolyte gating geometry, we drive the reversible (de)intercalation of oxygen ions, allowing for precise control over the oxygen non-stoichiometry across the full crystallographic range: from the fully oxidized perovskite to the reduced infinite-layer phase. We demonstrate that the introduction of Cobalt significantly alters the reaction pathway during the voltage-driven reduction process. While the pure SFO system transitions through the expected sequence of Perovskite à Brownmillerite à Infinite Layer, the SFCO films reveal a more complex trajectory due to the easier reducibility of Cobalt. This results in the stabilization of distinct intermediate states, specifically revealing distinguishable "oxidized" and "reduced" perovskite phases prior to the Brownmillerite transformation.

We further correlate these structural evolutions with their functional properties. For the SFO reference system, the phase transition is accompanied by a conductivity modulation of up to 5 orders of magnitude. By employing a setup that allows for simultaneous structural, electrical, and optical characterization, we provide a comprehensive map of the stoichiometry-functionality relationship for both materials. These results highlight the interplay between B-site chemistry and oxygen dynamics, offering new guidelines for controlling the redox pathways in future ionotronic devices.